The present invention relates to a system, method, apparatus and program for radio communication, and particularly to a system, method, apparatus and program for radio communication that make it possible to miniaturize a SAW tag and enable a tag reader to detect the SAW tag reliably in an RFID (Radio Frequency Identification) system using SAW (Surface Acoustic Wave).
RFID (Radio Frequency Identification) systems have been spreading recently. An RFID system includes a tag and a reader, and allows the reader to read information stored in the tag in a non-contact manner. The RFID system may be referred to as an ID (Identification) system, a data carrier system or the like. However, this system is often referred to as an RFID system (or abbreviated to an RFID) as a common name in various countries of the world. Thus, the system will be referred to as an RFID system also in the present specification. Incidentally, the RFID system is an “identification system using high frequencies (radio waves).”
A system using SAW (Surface Acoustic Wave), for example, is known as an RFID system.
A tag in the RFID system using SAW will hereinafter be referred to as a SAW tag. The SAW tag performs completely passive operations (passive operations effected by a tag reader), and therefore has a characteristic of allowing the operating distance thereof to be increased.
FIG. 1 shows an example of the configuration of such a SAW tag. As shown in FIG. 1, the SAW tag 1 has an antenna 11, an interdigital electrode 12, and reflection electrodes 13 to 15.
The antenna 11 is formed by, for example, a half-wavelength dipole antenna or the like. The antenna 11 transmits and receives radio waves of an operational frequency (communication carrier frequency) fo. A VHF (Very High Frequency) band or a UHF (Ultra High Frequency) band is generally allocated for the communication carrier frequency fo.
The interdigital electrode 12 has an electrode pitch d1 (=λ0/2) of half a wavelength λ0 of a surface acoustic wave. The interdigital electrode 12 excites the surface acoustic wave via a piezoelectric effect by a radio wave (high-frequency electric field) received by the antenna 11. That is, the interdigital electrode 12 supplies the radio wave received by the antenna 11 as the surface acoustic wave to the reflection electrodes 13 to 15.
The reflection electrodes 13 to 15 reflect the surface acoustic wave supplied from the interdigital electrode 12. That is, it can also be said that the reflection electrodes 13 to 15 emit a reflected wave in response to the surface acoustic wave. The reflected wave emitted from one of the reflection electrodes 13 to 15 is transmitted as a radio wave from the antenna 11 via the interdigital electrode 12.
The SAW tag 1 has unique data for identifying the SAW tag 1 itself. A reflection electrode (the reflection electrodes 13 to 15 in the example of FIG. 1) indicates that a predetermined piece of bit data including the unique data is “1.” Specifically, in the conventional SAW tag 1, a position representing each bit of the unique data is predetermined; when a reflection electrode is placed at a position corresponding to a predetermined bit, bit data of the bit is “1,” while when no reflection electrode is placed at the position, the bit data of the bit is “0.”
Thus, the unique data in FIG. 1 is 4 bits. The reflection electrode 13 is placed at a position representing the most significant bit (first bit); the reflection electrode 14 is placed at a position representing the next bit (second bit); the reflection electrode 15 is placed at a position representing the least significant bit (fourth bit in the example of FIG. 1); and no reflection electrode is placed at a position representing the third bit. Hence, the unique data is 4 bits of “1101.” In other words, FIG. 1 shows an example of the configuration of the SAW tag 1 when 4-bit data of “1101” is assigned as the unique data. That is, since the unique data of the SAW tag 1 differs for each apparatus type, the number of reflection electrodes and placement positions differ for each apparatus type.
The operation of a tag reader (not shown in the figure) recognizing the SAW tag 1 will next be described with reference to FIG. 2 and FIG. 3.
FIG. 2 is a timing chart showing the timing of transmission of check pulses and the timing of reception of reflected pulses in response to the check pulses in the tag reader.
Incidentally, in this case, a transmission pulse transmitted from the tag reader to check (detect) the unique data of the SAW tag (the SAW tag 1 of FIG. 1 in this case) is referred to as a check pulse. A pulse corresponding to a reflected wave occurring when a surface acoustic wave corresponding to a check pulse is reflected from a reflection electrode (the reflection electrodes 13 to 15 in the example of FIG. 1) is referred to as a reflected pulse.
FIG. 3 is a diagram showing the relationship between a surface acoustic wave corresponding to a check pulse and reflected waves in response to the surface acoustic wave (reflected waves corresponding to reflected pulses).
Suppose that, as shown in FIG. 2, for example, the tag reader transmits a check pulse 21 at a time t0.
The check pulse 21 transmitted from the tag reader passes through the antenna 11 and the interdigital electrode 12 in FIG. 1, and then arrives at each of the reflection electrodes 13 to 15 as a surface acoustic wave 41, as shown in FIG. 3. The arrived surface acoustic wave 41 is reflected by the reflection electrodes 13 to 15, and returned to the interdigital electrode 12 as reflected waves 51 to 53, respectively. That is, the reflection electrodes 13 to 15 emit the reflected waves 51 to 53, respectively.
The reflected waves 51 to 53 are each transmitted as a radio wave via the interdigital electrode 12 and the antenna 11. The tag reader receives the radio waves, and then detects the radio waves as a reflected pulse 31, a reflected pulse 32, and a reflected pulse 34, respectively, as shown in FIG. 2.
However, as shown in FIG. 3 (FIG. 1), since the placement positions of the reflection electrodes 13 to 15 differ from each other, the arrival times of the reflected waves 51 to 53 at the tag reader differ from each other.
Specifically, as shown in FIG. 2, the tag reader detects the reflected pulse 31 corresponding to the reflected wave 51 emitted by the reflection electrode 13 at a time t1 after the passage of a time T1 from a time to at which the check pulse 21 is transmitted. Thereby, the tag reader determines that the bit data of the first bit (most significant bit) in the unique data of the SAW tag 1 to which the check pulse 21 is transmitted is “1.”
Similarly, the tag reader detects the reflected pulse 32 corresponding to the reflected wave 52 emitted by the reflection electrode 14 at a time t2 after the passage of a time T2 from the time t0 at which the check pulse 21 is transmitted. Thereby, the tag reader determines that the bit data of the second bit in the unique data of the SAW tag 1 to which the check pulse 21 is transmitted is “1.”
On the other hand, a reflected pulse 33 does not arrive at the tag reader even at a time t3 at which the reflected pulse 33 next to the reflected pulse 32 is to arrive (the SAW tag 1 in FIG. 1 has no reflection electrode disposed to emit a reflected wave corresponding to the reflected pulse 33). Thereby, the tag reader determines that the bit data of the third bit in the unique data of the SAW tag 1 to which the check pulse 21 is transmitted is “0.”
Then, the tag reader detects the reflected pulse 34 corresponding to the reflected wave 53 emitted by the reflection electrode 15 at a time t4 after the passage of a time T3 from the time to at which the check pulse 21 is transmitted. Thereby, the tag reader determines that the bit data of the fourth bit (least significant bit) in the unique data of the SAW tag 1 to which the check pulse 21 is transmitted is “1.”
Thus, the tag reader can detect that the unique data of the SAW tag 1 to which the check pulse 21 is transmitted is “1101” and thereby recognizes the SAW tag 1.
A SAW tag 61 as shown in FIG. 4 is disclosed in L. Reindl and W. Ruile, Programmable Reflectors for SAW-ID-Tags, Siemens AG, Corporate Research and Development, Munich, Germany, 1993, Ultrasonic Symposium, pp. 125–130. As shown in FIG. 4, the SAW tag 61 has an antenna 11 and an interdigital electrode 12 having basically the same configuration and function as the SAW tag 1 of FIG. 1. However, the SAW tag 61 of FIG. 4 has reflection electrodes 71 to 74, which are interdigital electrodes having an electrode pitch d2 (=λ0/4) of ¼ of a wavelength λ0 of a surface acoustic wave, in place of the reflection electrodes 13 to 15 of the SAW tag 1 of FIG. 1.
Each of the reflection electrodes 71 to 74 functions as a reflector to reflect a surface acoustic wave when both terminals thereof are open. On the other hand, each of the reflection electrodes 71 to 74 does not function as a reflector and passes a surface acoustic wave when both terminals thereof are short-circuited. Thus, by changing a state of connection of both terminals of each of the reflection electrodes 71 to 74, it is possible to control the reflection and passage of a surface acoustic wave.
In FIG. 4, for correspondence with FIG. 1, each of the reflection electrode 71, the reflection electrode 72, and the reflection electrode 74 has both terminals thereof open to function as a reflector. That is, each of the reflection electrode 71, the reflection electrode 72, and the reflection electrode 74 reflects a surface acoustic wave supplied from the interdigital electrode 12, as with the reflection electrodes 13 to 15 in FIG. 1. In other words, each of the reflection electrode 71, the reflection electrode 72, and the reflection electrode 74 emits a reflected wave in response to the surface acoustic wave, and supplies the reflected wave to the interdigital electrode 12.
Since both terminals of the reflection electrode 73 are short-circuited, the reflection electrode 73 passes the surface acoustic wave supplied from the interdigital electrode 12. That is, the reflection electrode 73 does not emit a reflected wave in response to the surface acoustic wave (does not supply a reflected wave to the interdigital electrode 12).
Thus, the SAW tag 61 having the configuration of FIG. 4 operates in exactly the same manner as the SAW tag 1 having the configuration of FIG. 1. By performing basically the same processes as the series of processes described above, the tag reader can detect that the unique data of the SAW tag 61 having the configuration of FIG. 4 is also “1101,” and recognizes the SAW tag 61.
However, a reflection electrode in conventional SAW tags having the configurations of FIG. 1 and FIG. 4 functions to represent one predetermined piece of bit data included in the unique data. Therefore, as the number of bits of the unique data is increased, the number of reflection electrodes needs to be correspondingly increased by the same number. As a result, as the number of bits of the unique data is increased, the physical size of the SAW tag becomes larger.
In the example of FIG. 2, although the amplitude of the reflected pulses is shown as being the same as the amplitude of the check pulse (transmission pulse from the tag reader) for simplicity of description, in practice, the more distant the position of the reflection electrodes is from the interdigital electrode (interdigital electrode 12 in FIG. 1 and FIG. 4), the more the amplitude of the reflected pulses will be attenuated. That is, the amplitude of the reflected pulse corresponding to the surface acoustic wave reflected (that is, the reflected wave emitted) by the reflection electrode disposed at the position corresponding to the least significant bit of the unique data (for example, the reflection electrode 15 in the example of FIG. 1 and the reflection electrode 74 in the example of FIG. 4) will be attenuated greatly. As a result, the larger the number of bits in the unique data of the SAW tag (that is, the more distant the reflection electrode is from the interdigital electrode), the more difficult it is for the tag reader to recognize the SAW tag (to detect bit data of the least significant bit of the unique data).